Mirror filters are used in subband coding (SBC) schemes. Subband coding has been shown to be an effective method of reducing the bit-rate required for the transmission of telephone band signals (R. E. Crochiere, S. A. Webber and J. L. Flanagan, "Digital coding of speech in subbands", Bell Sys Tech J. Vol. 55 pp 1069-1085 (1976 Oct)). It is a waveform coding technique that can cope with signals from a wide variety of sources, and could, therefore, prove useful in the public switched network. Coding efficiency is achieved by virtue of the fact that the individual subbands can be encoded using differing coding strategies, and these can be optimized to the statistics of the input signals. The method is especially applicable to speech transmission because the coders can be made to exploit certain perceptual effects connected with hearing. In particular, provided that appropriate quantizers are used, the technique will result in the quantization noise at the output of the codec having a similar power spectral distribution to that of the uncoded signal; it is well-known that the human ear is relatively tolerant to noise in the parts of the spectrum occupied by high level wanted signals. Additionally, the higher frequency components can be represented with reduced accuracy because the ear is less sensitive to their absolute content.
Reducing the number of bits does mean, however, that the measured quantization noise in the SBC channel will be of higher level than the channel noise of a normal telephone connection. Nevertheless, when using appropriate coding schemes, the subjective quality of the SBC output signal will remain good. In fact, listening tests suggest that transmission rates can be as low as 16 kbit/s before degradations become noticeable.
Unfortunately, simple tests, such as these, do not highlight problems that could occur during 2-way transmission, with all the additional impairments that can be expected within a telephone network. For example, there are many telephone connections which introduce propagation delays close to the CCITT recommended maximum (recommendation G122). If SBC systems are added to these circuits, and if the band-splitting filters introduce significant extra delay, then these connections could become prone to observable echoes. Non-blocking echo cancellation equipment will be of minimal use in these situations, because the cancellation algorithms behave poorly in the presence of high level channel noise. For this reason a study was carried out into the feasibility of building band-splitting filters that would result in SBC systems introducing less signal delay.
Prior proposals have employed quadrature mirror filters (QMF) with a linear phase response (i.e. a constant delay).
According to the present invention we provide a filter arrangement comprising a pair of first and second filters for passing respectively frequencies above and below a transition frequency, in which the amplitude responses and the delay responses of the filters are mirror images about the transition frequency, the filters are not quadrature filters and the delay response with respect to frequency of each filter is non-constant. Since the filters are not quadrature filters, the delay response is asymmetrical about the transition frequency.
In filters employing a linear phase response, the delay is not a minimum and delays add arithmetically in cascaded systems. The present proposal permits a reduction of the delay in each filter, and in multi-band cascaded systems the delay peaks can be arranged not to coincide in subsequent filters. Preferably the filters are minimum phase response filters (i.e. the z-plane plot contains no zeros outside the unit circle).
In the past, quadrature filters have been employed viz the phase shifts introduced by the two filters of the pair always differ by 90.degree.. Recognizing, however, that this represents an unnecessary constraint, in the invention the first and second filters are not required to have a phase difference of 90.degree..
It is preferred that the peaks of the delay response of each filter lie beyond the transition frequency: in this way, in a system in which a signal is divided in a first pair of filters, subsampled, reconverted to the original sampling rate and filtered by a second pair, the peak delay of the signal path through the system can be less than that of the two channels considered separately.
The amplitude response of the system can be made arbitrarily flat--subject of course to constraints in terms of the number of filter stages permissible whilst meeting desired delay limits. The phase response of course will not be flat. For speech transmission this is relatively insignificant: for other applications, phase equalization can be employed as necessary.
Preferably the filters are recursive: although not essential, this is highly desirable from the practical viewpoint to reduce the number of delay stages.
In another aspect, the invention provides a sub-band coding apparatus having an input for a sampled input signal, first and second filters of the type described above arranged to filter the sampled signal, and means for down-sampling the filtered outputs; and also a decoding apparatus comprising means for restoring the sampling rates of sub-band input signals, first and second filters of the type described above for filtering the restored signals, and means for combining the filter outputs to produce a decoded signal.
The manner in which the samples may be encoded/decoded at the coder output/decoder input, in terms of the number of bits and coding schemes used, for providing a reduction in the transmitted bit rate is not material to the present invention and conventional techniques can be employed.
Of course, the use of only two sub-bands is the minimum case, and more sub-bands may be employed. Although it would be possible to employ band-pass filters for this purpose, cascaded filter pairs are more commonly used to divide the band of interest progressively into two, four, eight etc. sub-bands, and this method may be used in the context of the present invention.
Assuming Nyquist sampling at the input, it will normally be necessary to divide the band into two equal parts, so that, for a sampling rate f.sub.s, with a system bandwidth of f.sub.s /2, the transition frequency of the first filter pair would be f.sub.s /4.
Preferably the filter parameters are selected such that the overall delay response of the system is, over a substantial portion of the system bandwidth, substantially equi-ripple (i.e. that the peak-to-peak ripple is substantially constant) and the spacing of the peaks of the delay response is substantially uniform. Generally these peaks will occur at the band transitions. As a consequence, equalization can readily be carried out by means of a relatively simple polyphase all-pass filter.